The present invention relates generally to improvements in camera film advancing devices and it relates more particularly to an improved film advancing device of the type which responds to the sensing of the completion of advance of an indexed length of film to restrain any excess advace.
Mechanisms of the subject nature are known in the prior art, including a first type in which the film advancing mechanism is braked, during the film advancing operation in response to the completion of advance of a predetermined length of film to thereby stop the film advance. This first type of device is described in U.S. Pat. No. 3,736,854 issued on Jan. 5, 1973 and U.S. Pat. No. 3,748,991 issued on July 31, 1973. With this first type of device, however, the film advance mechanism is suddenly or abruptly restrained in the course of its operation, greatly impairing the smooth operation of the mechanism, and at the same time, imparting an excessive force or shock to the component parts of the mechanism thereby resulting in an early malfunction or damage to the mechanism and greatly reducing its reliability and durability. A second type of device which is described in U.S. Pat. No. 4,009,478 issued Feb. 22, 1977 is so constructed that a clutch mechanism interconnecting a film windup member with a film takeup shaft is disabled to cut off the drive transmission between the two, upon completion of advance of a predetermined length of a film, thus stopping the film advance. Although this construction may eliminate the disadvantages of the first type of device, the second type is unstable in function in that the clutch may not be smoothly or is never reset to an operative condition in which the clutch interconnects the wind-up member and take-up shaft. A third type of mechanism which is described in U.S. Pat. No. 3,829,875, issued Aug. 13, 1974 uses planetary gears to disable the film advance mechanism during operation in response to a signal sensing the completion of an advance of a predetermined length of film, thereby stopping the film advance. Instead of restraining the movement of the film windup member, this type of mechanism absorbs the excess drive movement, thereby eliminating the disadvantages of the first type. However, a large number of gears are required for this type of device, thus resulting in a complex construction. In addition, a fourth type of mechanism uses a coil spring through which the film windup member and the film takeup shaft are drive coupled to transmit power from the film wind-up member to the film takeup shaft through the action of the tightening of the coil spring to advance the film, and to open or cut off the drive transmission through the loosening of the coil spring when a predetermined length of film has been advance, i.e., when a film frame advance has been completed, thus preventing an excess of film advancing. This type of mechanism is advantageous in that the film advance is stopped without restraining the movement of the film wind-up member to thus provide smooth film advance operation, the power or drive transmission or cutoff can be controlled through the tightening or loosening of the coil spring to assure functional stability, and only a coil spring is required as the power transmission or cutoff controlling means permitting a very simple construction.
The present invention relates to improvements in the above-identified fourth type of film advance mechanism or device, which type is described in Japanese Utility Model Publication No. 45-9355 issued on May 1, 1970. The device described in the above publication employs a construction in which a coil spring is wound around a film-advance shaft integrally rotatable with a wind-up lever, one free end of the coil spring being fixed to a spool shaft and the other end to a stop gear rotatably fitted to the shaft of the wind-up lever. In this device, the coil spring tightens on the wind-up shaft in response to the shaft rotation, and thus the coil spring is rotated integrally with the film advance shaft. The rotation of the coil spring is transmitted through its free end to the spool shaft by which the film is advanced. When a predetermined length of film has been advanced, the stop gear is restrained to stop the other end of the coil spring. Because the coil spring is rotated, this stopping produces an acting force to loosen the coil spring. When this end of the coil spring is stopped, therefore, the coil spring is immediately loosened to stop its rotation, cutting off the drive transmission from the wind-up shaft to the spool shaft and thus causing the film advance to be stopped. With the aforesaid device, however, the power transmission from the wind-up shaft as a drive member to the spool shaft as a driven member is effected through the free end of the coil spring alone, the device thus posing a problem in durability. In other words, this device is constructed such that the spool shaft is rotated by the free end bent radially from the coil spring, and with a greater load applied to the spool shaft or after long use of the device, the bent end of the coil spring may be broken or deformed to disengage the spool shaft, thus disabling the power transmission.